1. Field of the Invention
The present invention relates to absorbent articles typified by sanitary napkins, paper diapers and pads for incontinent people. More particularly, the present invention relates to absorbent articles having a high absorbency and improved properties of absorbing body fluids, particularly absorption velocity, diffusing capacity, inhibition of reflow of the fluid from the absorbent element and inhibition of leakage.
2. Description of the Prior Art
Various absorbent articles have been proposed for the purpose of improving the absorption of body fluids and various improvements have been developed. Most of the improvements relate to an improvement in the absorption velocity, inhibition of reflow of the fluid from the absorbent element to the surface, prevention of leakage and reduction of stickiness to the body.
For example, as for the material of the absorbent, it was proposed to absorb and retain the fluid in a highly absorbent polymer by a physicochemical effect, i.e. by ionic osmotic pressure, thereby improving the absorption capacity and also preventing the reflow of the fluid instead of using hydrophilic absorbent paper or pulp which absorbs and retains the fluid in physical fine spaces. In fact, the absorbency was improved by this proposal and at present, an absorbent element comprising a combination of pulp and a highly water-absorbent polymer is used for most of the absorbent articles. However, the prevention of leakage, even by such absorbent articles, is still insufficient, as suggested by the fact that the main reason for dissatisfaction of the absorbent articles resides in the leakage. Namely, the body fluid absorption velocity of the highly water-absorbent polymer which absorbs and retains fluid by ionic osmotic pressure is limited and, in addition, no body fluid can be absorbed unless the polymer is wetted with the body fluid. Therefore, the highly water-absorbent polymer must have been used in combination with pulp or the like having a high water absorption velocity. However, the pulp used in combination with the polymer is extremely reduced in its strength when it is wetted and the strength is not recovered, though it exhibits some compression and bending recoveries when it is dry. When stress is applied to the wet pulp, compression deformation (hereinafter referred to as the twist) thereof is caused to seriously reduce the absorption space. In addition, the body fluid once absorbed is easily pressed out by the twist and reabsorption velocity after the twist is seriously reduced. Thus in such a case, the absorbent element is twisted before the highly absorbent polymer sufficiently exhibits its absorbency and the twist frequently causes the leakage.
Techniquies of making up such defects of the absorbent elements are proposed in, for example, Japanese Utility Model Publication No. 6099/1981, Japanese Patent Laid-Open No. and 22854/1986 and Japanese Patent Laid-Open No. 97748/1981.
(1) A sanitary napkin disclosed in Japanese Utility Model Publication No. 6099/1981 is composed of a liquid-permeable surface material 1, an absorbent element 2 mainly comprising hydrophilic fibers and a liquid-impermeable back sheet 3 as shown in FIG. 5, and an elastic porous element 4 (such as a foam) having shape-retaining properties is provided in the absorbent element 2 or at the rear of it in order to prevent the absorbent element 2 from being deformed.
(2) A sanitary napkin disclosed in Japanese Patent Laid-Open No. 22854/1986 is the same as the sanitary napkin (1) except that the hydrophilic fibers used as the absorbent element 2 are replaced by an elastic porous element (such as urethane skeleton foam or polyester fiber aggregate bonded with a binder).
(3) A sanitary napkin disclosed in Japanese Patent Laid-Open No. 97748/1981 comprises hydrophilic fibers and/or water-absorbent polymer uniformly dispersed in the cells of an elastic porous element 4 used as an absorbent element 2 as shown in FIG. 7.
Although the shape of the sanitary napkin (1) is retained as a whole, the elastic porous element 4 arranged in parallel with the absorbent element 2 has insufficient hydrophilic properties and its skeletal structure is not effective in absorbing or retaining the body fluid and, therefore, it has substantially no absorbent or retentive properties per se. In addition the absorbent element 2 per se mainly comprising the hydrophilic fibers has no recovery properties. Therefore, when stress is applied to the sanitary napkin, the twist and permanent set in fatigue of the absorbent element 2 are caused to make the exhibition of a sufficient absorbing effect of the absorbent element 2 impossible.
In the sanitary napkin (2), although the elastic porous element used as the absorbent element 2 has excellent recovery from deformation, the liquid retentivity thereof is poor as in the napkin (1) and such an absorbent article is practically useless for this purpose.
Although the sanitary napkin (3) has liquid absorbency and liquid retentivity superior to those of the napkin (2) to some extent, its performance is yet insufficient for use as the absorbent article, since the liquid retentivity of the elastic porous element which is the main component of the absorbent element 2 is quite poor.
Under these circumstances, the absorbent articles comprising a combination of the above-described pulp and highly absorbent polymer are now mainly used.
Various ideas were proposed for inhibiting the backing of the liquid with a surface sheet even after the reflow of the liquid from the absorbent element in the currently mainly used absorbent articles. The surface sheet is made of a nonwoven fabric comprising a hydrophobic material (such as polypropylene staple, polyethylene staple or polyester staple), instead of an ordinary hydrophilic material (such as rayon staple). In such a case, a space of hydrophobic atmosphere is formed between the surface of the user's body and the absorbent element to reduce the stickiness of the surface. In particular, absorbent articles in which the surface sheet is made of a hydrophobic sheet (film) having open cells were recently proposed. Since the hydrophobic material having open cells is used as the surface sheet, the liquid is not retained in the surface sheet and it is effective in preventing sticking. However, the liquid permeability is a requirement contradictory to the prevention of the reflow of the liquid and these two requirements cannot be completely satisfied at the same time. Namely, as the space of the hydrophobic atmosphere is increased, the open cell part is decreased to ensure the prevention of the reflow of the liquid but the permeation of the liquid is inhibited. Thus by merely adjusting the size of the space, the permeability to the liquid and the prevention of the reflow of the liquid cannot be easily and effectively attained at the same time.